1. Field of the Invention
This invention relates to the field of dentistry and particularly dental knives for trimming thermoplastic dental trays.
2. Related Art
Thermoplastic dental trays are used in the application of dental fluoride, dental bleaching, and other like dental treatments. The dental tray is sometimes vacuum formed on a full size model of a patient's jaw ridge, which is the ridge-formed by parts of the gums, the adjacent teeth and the alveolar portions of the jaws, in the areas to receive the dental treatment. A mold of soft plastic material which hardens into such a full size model is obtained by taking an impression of the appropriate area of the patient's jaw ridge. The appearance and the fit of the finished dental tray depends to a large degree on the precision of the model on which the tray is formed and subsequently depends on the precision of the method of trimming the tray. The initial dental impression tray that is used to make the full size model is usually formed from a thermoplastic sheet. The initial impression tray closely approximates the shape of and size of the specific patient's jaw ridge for use as a mold in the manufacture of an accurate full size study model on which a more precise and final dental tray can be formed for use in a dental treatment. The creation, by vacuum forming, of a final hardened thermoplastic dental tray to be used for the fluoride, bleaching or other dental treatment requires trimming of the excess thermoplastic from the dental tray material to produce a tray with the desired shape to match the patient's gingival line. A trimming of the dental tray that matches the gingival line should produce a tray with a scalloped edge that matches the gingival line. Trimming or cutting is typically accomplished by using scissors with curved shear blades. However, there is a problem with a scissors trimming method. This method of trimming or cutting with scissors can only be done after the tray is removed from the mold and thus is very inaccurate. Once the tray is removed from the full size model, one must trim the tray utilizing the trace lines that remain as an impression from the model. There is great difficulty when scissors are used to accurately trim along the trace line impression left by the gingival line of the model in order to obtain the final tray to be used for application of the dental treatment. An additional problem is the difficulty in trimming the material with scissors once the material has fully cooled and hardened. The thermoplastic is substantially tough at body or room temperatures. Therefore when cut with scissors, the thermoplastic may stretch due to the shearing action of the scissors and this stretching may result in an oversize or misshapen dental tray. The accuracy of the trimming is key to the effective manufacture of a good dental tray for application of a fluoride or bleaching dental treatment. An accurate trimming process produces a dental tray that will have a tight and snug fit around the teeth to be treated. A tight and snug fit is desired for the most effective treatment. A dental tray that closely follows the shape of the patients gum line, 1 mil shy of gingival line, is desirable for the snug fit which promotes an effective treatment. When scissors are used, usually the distance between the gingival line and the dental tray is much greater than 1 mil and the tray thus overlaps the gum.
Also, trimming and cutting with scissors many times yield poor result such as a jagged edge which is not comfortable to the patient and many cuts or trims are not easy to accurately perform with scissors. Cutting with scissors is inherently a series of cuts, which may or may not align with each other. If not, a resulting jagged edge is probable. This result either makes for a jagged edge or loose fit or the tooth is not treated up to the gingival line.
Neither of the above results will provide an optimum treatment.
There are also heated dental knives that are utilized to trim thermoplastic dental trays. These heated knives have electrically powered heating elements that heat the blade tip. An adjustable heat setting is provided. The adjustable heat setting is usually provided by adjustment of a potentiometer which varies the voltage across a transformer and resulting power to the heating element. However, a problem with such heated dental knives is that there is no real time monitoring of the blade tip temperature, thus no feedback is provided for dynamic temperature control. There is only discrete voltage level adjustment capability. This type of heated knife will allow the actual blade cutting tip to over-shoot or under-shoot the optimal temperature if the potentiometer is set to the incorrect level. Adjusting the potentiometer to an optimal setting for trimming thermoplastic dental trays is accomplished by trial and error. Over-shooting the desired temperature will result in melting the thermoplastic. If melting occurs the thermoplastic will be deformed and possibly the severed thermoplastic will immediately meld back together after cutting. If under-shooting occurs the thermoplastic will possibly tear or cut unevenly instead of providing a cleanly seared cut. Temperature control at the blade tip is important because the heating element of dental knives such as the one described above are not designed to maintain a stable temperature range. For a given potentiometer setting, the temperature at the blade tip may vary widely. The ability to adjust the power level input to the heating element of the dental knife over a large temperature range is not needed or desired for the subject application because it allows more room for error. User error occurs when the level control is set to the wrong level. Also, there is a risk for error due to component anomalies such as failure of the potentiometer and failure the transformer assembly. In addition it is more difficult to design a dental knife that can maintain a stable temperature range at a given level setting when the power input to the heating element can be varied over a large range. There is not the ability to thermally design a dental knife that is optimized to maintain a temperature range for the fixed heating element power input and output needed for a dental tray knife.
Another difficulty with the current dental knives is that they are not ergonomically designed. Specifically the blade tip is too long for the comfortable application of trimming thermoplastic dental trays.
There are some prior art patents that address the forming, handling, or cutting of thermoplastic items. None are designed to perform the trimming or cutting of thermoplastic dental trays and none, if utilized, will yield the result of a dental tray that has a snug fit and closely follows the gingival line. For example, U.S. Pat. No. 5,046,251 issued Sep. 10, 1991 to Scott shows a portable lightweight tool employing a heating element and configured in a scissor-like embodiment whereby at least one blade of the scissors employs a heating element. A combination cutting and searing is achieved by the normal scissor-action of the embodiment. However, this item was designed to cut fabric material with high thermoplastic content and to be used by a seamstress, upholster, or the like. Scissors with one heated shear may make for ease of cutting, assuming the heating element achieves a high enough temperature to sear dental thermoplastic when the thickness is considered in comparison to that of fabric. However, as noted above, cutting dental trays with scissors makes jagged edges due to the discontinuance nature. Also, cutting with insufficient heat will result in tearing and stretching of the thermoplastic and with too much heat results in resealing of the cut. In addition, the scissors-like embodiment will not solve the problem for the trimming required in areas that are difficult to trim with scissors. A more practical solution to the continuing problem of jagged dental trays is needed.